The development of high energy battery systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly reactive anode materials, such as lithium, sodium, and the like and the efficient use of high energy density cathode materials, such as manganese dioxide. The use of aqueous electrolytes is precluded in these systems since the anode materials are sufficiently active to react with water chemically. It has therefore been necessary, in order to realize the high energy density obtainable through use of these highly reactive anodes and high energy density cathodes, to turn to the investigation of nonaqueous electrolyte systems and more particularly to nonaqueous organic electrolyte systems.
The term "nonaqueous organic electrolyte" in the prior art refers to an electrolyte which is composed of a solute, for example, a salt or complex salt of Group I-A, Group II-A or Group III-A elements of the Periodic Table, dissolved in an appropriate non-aqueous organic solvent. Conventional solvents include propylene carbonate, ethylene carbonate or .gamma.-butyrolactone.
A multitude of solutes is known and recommended for use but the selection of a suitable solvent has been particularly troublesome since many of those solvents which are used to prepare electrolytes sufficiently conductive to permit effective ion migration through the solution are reactive with the highly active anodes mentioned above. Most investigators in this area, in search of suitable solvents, have concentrated on aliphatic and aromatic nitrogen-and oxygen-containing compounds with some attention given to organic sulfur-, phosphorus-and arsenic-containing compounds. The results of this search have not been entirely satisfactory since many of the solvents investigated still could not be used effectively with high energy density cathode materials, such as manganese dioxide (MnO.sub.2), and were sufficiently corrosive to lithium anodes to prevent efficient performance over any length of time.
Although manganese dioxide has been mentioned as a cathode for cell applications, manganese dioxide inherently contains an unacceptable amount of water, both of the adsorbed and bound (absorbed) types, which is sufficient to cause anode (lithium) corrosion along with its associated hydrogen evolution. This type of corrosion that causes gas evolution is a serious problem in sealed cells, particularly in miniature type button cells. In order to maintain overall battery-powered electronic devices as compact as possible, the electronic devices are usually designed with cavities to accommodate the miniature cells as their power source. The cavities are usually made so that a cell can be snugly positioned therein thus making electronic contact with appropriate terminals within the device. A major potential problem in the use of cell-powered devices of this nature is that if the gas evolution causes the cell to bulge then the cell could become wedged within the cavity. This could result in damage to the device. Also, if electrolyte leaks from the cell it could cause damage to the device. It is therefore important that the physical dimensions of the cell's housing remain constant during discharge and that the cell will not leak any electrolyte into the device being powered. Consequently, the manganese dioxide for use in this invention has to be heat treated to lower its water content to less than 1 percent by weight as is known in the battery art.
U.S. Pat. No. 4,133,856 discloses a process for producing a MnO.sub.2 electrode (cathode) for nonaqueous cells whereby the MnO.sub.2 is initially heated within a range of 350.degree. to 430.degree. C. so as to substantially remove both the adsorbed and bound water and then, after being formed into an electrode with a conductive agent and binder it is further heated in a range of 200.degree. to 350.degree. C. prior to its assembly into a cell. British Pat. No. 1,199,426 also discloses the heat treatment of MnO.sub.2 in air at 250.degree. to 450.degree. C. to substantially remove its water component.
Propylene carbonate and dimethoxyethane have been employed as solvents for a wide range of solutes as disclosed in U.S. Pat. No. 4,158,723. Specifically, this patent discloses a Li/PbO cell containing Bi.sub.2 O3, Sb.sub.2 O.sub.3 or SnO.sub.2 additive in the PbO cathode and an electrolyte of a mixture of an ether (e.g. dimethoxyethane) and an ester (e.g. propylene carbonate) with a solute such as lithium trifluoromethane sulfonate.
While the theoretical energy, i.e., the electrical energy potentially available from a selected anode-cathode couple, is relatively easy to calculate, there is a need to choose a nonaqueous electrolyte for a couple that permits the actual energy produced by an assembled battery to approach the theoretical energy. The problem usually encountered is that it is practically impossible to predict in advance how well, if at all, a nonaqueous electrolyte will function with a selected couple. Thus a cell must be considered as a unit having three parts: a cathode, an anode, and an electrolyte, and it is to be understood that the parts of one cell are not predictably interchangeable with parts of another cell to produce an efficient and workable cell.
It is an object of the present invention to provide a nonaqueous cell employing among other components a manganese dioxide-containing cathode wherein the water content is less than 1 weight percent based on the weight of the manganese dioxide and a stable electrolyte solution of lithium trifluoromethane sulfonate dissolved in propylene carbonate and dimethoxyethane which permits excellent cathode utilization over a wide range of temperatures.
It is another object of the present invention to provide a heat-treated manganese dioxide nonaqueous cell employing a lithium anode.
It is another object of the present invention to provide a lithium/MnO.sub.2 nonaqueous cell employing a liquid organic electrolyte consisting of lithium trifluoromethane sulfonate dissolved in propylene carbonate and dimethoxyethane.